Re: Nested iterators, how to best implement?
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- Subject: Re: Nested iterators, how to best implement?
- From: Sean Conner <sean@...>
- Date: 2015年1月29日 15:27:56 -0500
It was thus said that the Great Tony Papadimitriou once stated:
> Hi all,
>
> I’m looking for the best (?) general way to implement *nested* iterators
> in Lua (5.3, of course).
>
> For example: I have an iterator that returns single items when used in a
> FOR .. IN loop. Each item is treated somehow but that loop, and the
> result I want it to become available in another iterator.
>
> Easy, but I do NOT want to store all intermediate results in a table, and
> then traverse the table again, as the size of the table could be too large
> or even unknown while the iterator is running (until some condition
> happens that terminates it).
>
> In Python 3, for example, this would be very simple with each routine
> using YIELD instead of RETURN, and iterators can be nested to an arbitrary
> level. But, in Lua, I can’t seem to find an obvious (memory efficient)
> solution. (I would love for Lua to have the same simplicity in this
> regard.)
>
> Perhaps explicit use of co-routines is one solution, but maybe there is a
> more immediate way I missed.
>
> Maybe someone can point me to some example.
Here's an example that I think answers what you are asking:
function iterator(a,b)
return coroutine.wrap(function()
for ai = 1 , #a do
for bi = 1 , #b do
coroutine.yield(string.format("%s-%s",tostring(a[ai]),tostring(b[bi])))
end
end
end)
end
A = { 1 , 2 , 3 , 4 , 5 }
B = { "one" , "two" , "three" , "four" , "five" }
for item in iterator(A,B) do
print(item)
end
-spc